In order to output a color image on an image-carrying medium such as a sheet of paper, color-rendering material or colorant such as toner or ink is generally applied onto the image-carrying medium such as a white image-carrying medium. For a human to perceive the rendered color image, light must be shined towards the image carrying-medium. Since more light is absorbed by the applied colorant than the white image-carrying medium, the light reflected from the applied colorant is perceived as a color. Because of this absorbing nature, the above color-rendering process is known as a "subtract," and subtractive primary colors include cyan (C), magenta (M), yellow (Y) and black (K). For example, using color toner, a particular mixture of four substantially distinct primary color toner (i.e., CMYK toner) renders a desired color. In contrast to the above described subtractive system, the additive primary colors such as red (R), green (G) and blue (B) are used to generate an image while generating light such as a television screen or cathode ray tube (CRT).
To duplicate a color image, the color image is generally scanned into RGB data via an input device and converted into CMYK data for an output device. For example, color copiers have an input device such as a scanner for scanning a color image before reproducing the color image on an image-carrying medium via an output device such as a printer. To convert the RGB scanned data into the CMY data, the scanner must be calibrated to produce R=G=B=1 for a standard white color and R=G=B=0 for a standard black color. In addition, the scanner must be also balanced for any achromatic gray input to generate R=G=B. Under the above described conditions, the RGB data is converted to the CMY data based upon the following relations: EQU 1-R=C.sub.p (1) EQU 1-G=M.sub.p (2) EQU 1-B=Y.sub.p (3)
where C.sub.p, M.sub.p and Y.sub.p are respectively ideal pure cyan, magenta and yellow toner. As briefly noted above, the CMYK data has four separate values including a C value, a M value, a Y value and a K value for each pixel in the output image. Although the four separate values respectively specify an intensity or an amount of colorant for each pure primary color to achieve a desirable color on an image-carrying medium, the precise mixture of the primary colorant is more complicated than these CMYK values indicate for the following reasons.
Each primary colorant is not generally pure and includes other primary colorants. For example, referring to FIG. 1, cyan toner includes not only cyan color component but also some magenta and yellow color componet. The X axis indicates an output intensity level which specifies the amount of the cyan toner to be outputted from an output device to an image-carrying medium. The output intensity level is also known as a printer command intensity value for a particular primary color. The Y axis indicates a measured separate CMY color component levels of the outputted cyan toner which is measured by an input device such as a scanner. The composition of the CMY primary colors varies over the output intensity range. In other words, referring to FIG. 2, the composition of the cyan toner is shown over the same intensity value range when the cyan component of the cyan toner is used as a standard value of 1. The yellow and magenta components of the cyan toner is expressed with respect to the above standard value.
Referring to FIG. 3, the composition of magenta toner is illustrated in a similar fashion as the cyan toner. Magenta toner includes not only magenta toner but also yellow color component as well as cyan color component, and the composition of the CMY primary toner varies over the output intensity range. Referring to FIG. 4, the composition of yellow toner is illustrated in a similar fashion as other primary color toner. Although the yellow toner is substantially more pure than magenta and cyan toner, it still includes some magenta and cyan, and the composition of CMY toner also varies over the output intensity range.
Referring to FIG. 5, in order to render a desirable color, the above described CMY toner are superimposed. The colorant composition of the above described superimposed CMY toner also varies over an intensity range. Because each of the CMY toner is impure, the combined composition ratio of the superimposed or mixed CMY toner is not substantially identical among the three colorant components. Furthermore, the degree of the above described impurity of toner varies among manufacturers, toner products and even lots within the same product. Lastly, the relative composition of the toner may be unstable over an extended period.
The above described factors make it difficult to duplicate a consistently faithful color image using the colorant such as CMYK toner. To render a faithful color image, prior art attempts include various ways to manually adjust the CMYK intensity values based upon the observations of output images. These prior attempts using manual adjustments are disclosed in the following U.S. Pat. Nos.: 4,500,919 for adjusting a color development system using a CRT; 4,590,515 for increasing black toner in proportion to a reduced amount of CMY ink; 4,841,360 for adjusting based upon an image-carrying medium; and 4,845,550 for using six color signals to adjust coefficients for performing a color correction. The above prior attempts are generally imprecise and require operators some special training in color adjustments. Because of the above described non-uniform toner characteristics, even the experienced operators are not able to fully correct the problems.
To correct the color output in a more systematic and precise manner, other prior attempts include automatic adjustments. Japanese Patent 4-277974 discloses a method and a system for automatically adjusting the magenta value and the yellow value for the corresponding cyan value based upon the use of the gray scale. Furthermore, Japanese Patent 2-76760 also discloses a method and a system for automatically adjusting the output CMYK values based the input values of a predetermined gray scale chart which have been printed by an output device and read by an input device of the system to be adjusted.
Despite these efforts, none of the above described prior art addresses the problems associated with the impure primary colorant in color balancing. In other words, the above described prior art assumes that the colorant is substantially pure and color outputs are not affected by the impure nature of the colorant. Furthermore, because the colorant has the above described non-uniform composition over the intensity range, prior art does not effectively correct the color output intensity values for specifying a mixture of the CMYK rendering material.